I have the shelf made and coloured properly to match cabinets, now what
to use for a clear coat? *Poly, varnish, shellac or ?

This application SCREAMs for poly. No way Shellac. Any fluid with any
amount of water, ie vinegar, sitting on it will ruin it. It is not
ONLY harmed by alcohol. Lots of solvents and any wetness will harm it
significantly.

Oil varnishes will provide some varying degree resistance from some to
none. Poly is bullet proof except for too much heat, as will be any of
the finishes and it also will have the most resistance to heat of any.

Three choices. Poly, poly or poly. Or expoxy.